Thermal switch



y 20, 1943. B. N. CLACK 2,324,907

THERMAL SWITCH Filed July 22, 1941 lnvervbor't Ba iL Noel CLack, b ,MM

His Af'tor-neg.

Patented July 20, 1943 li silfloe Cla k; 'Wei ;;E lawl a sisnq t8-General Electric Company, a corporation of New York entire-lemmin 1e41seeratha ie JnrGrcat Britain AugustZZ, 194i) "roman. (01.250-275) I itention r lates t therma l ppei'a ed swi ch e 'thwtym whe ei aitherme ys s tiv leme t, o example :a bimetal, is heated b e low dis har Suchswic sarenow se o con ol n th -s.tar i g re fluorescent dis-- ha e ampsihavin el ctrodes which ,require 'h t eabeio s a tina d s harg :th re eeterm n l of a fiurr u s u fo th i h Attenajimesuihcient toraisethelamplelec'trcdes to their operating temperature, the series connectiontherebetweenisprolien by the opening of the switch contacts, due to thecooling and consequent flexing of the bimetal, but the glow dischargedoes not start again in the switch if the lamp has started because thevoltage then across the switch is not the full supply voltage, but theoperating voltage of the lamp. It is necessary, therefore, that thestarting potential of the switch should be less than the voltage of thecurrent source, but greater than the operating voltage of the lamp.

I have observed that switches of the kind de scribed above, which areperfectly satisfactory when exposed to light, have higher startingpotentials than the voltage of the current source when they have been inthe dark for some time and, thus, are useless for starting dischargelamps when the switch is inthe dark.

The object of my invention is to provide a glowtype thermal switchhaving approximately the same starting potential in the dark and in thelight. Other objects and advantages of my invention will be apparent tothose skilled in the art from the following description and from theappended claim.

I have discovered that an exceedingly small quantity of radioactivematerial capable of emitting alpha-rays is effective for maintaining thestarting potential of thermal switches of the glow discharge typesubstantially the same in the dark and in the light when said materialis mounted in such position within the switch envelope that thealpha-rays can reach the discharge path between the electrodes.

While I prefer to use radioactive uranium and thorium compounds for thepurposes of my invention, this preference is based largely on theavailability and comparatively low cost of these maerials and I r qemnle e th t the za ph tav emittin ma erial :s ch as med um chloride, 10, oth rsui abl eom eun .9 r dium-m h se I9 attainin vthedes red r sultIf th envelene ma o uranium slasaw ueh a 'the id sqlc d n m impendin ealicatieaserie N 13 19.03 filed'Ma e fi... .9 1 ,o wh ht is a l cationis a c inua ipnii zpa th uranium i th a e t e g ss next to t nte io ofthe en el pe is s ficient Accordingl heireii acti mateia i t-sa d ib c,in di t en e o fit sls -l cated thati t i. StQRGe-fE- m th d s har pa iessithanith i an of theelnh -r s n the interven nemat a :lip e e zt iplaea f w milli ra s o uranium o id :in: .enve1Ci e i u m itien thatth-,a1nh -,r ys emitted t ereb here t h d sen age Pa h wi hou pes netpugh;a ei su st ne n; essa n ee eieee n en qfo e ha t of thisspecification, a species of thermal switch of the glow discharge type isshown in a side elevational view.

Referring to the drawing, the glow-type thermal switch illustratedcomprises a sealed, tubular envelope ID of glass having a glass stem isfused thereto. Said envelope [0 is about 50 mm. in length and about 18mm. in internal diameter and has a gaseous atmosphere therein consistingof helium mixed with approximately 0.15 per cent of argon at a pressureof approximately 45 mm. mercury column. Two bimetallic strips I2 and [3,having contacts l4 and 15 attached to the end thereof remote from thestem l6, are mounted in parallel positions in said envelope [0. Saidstrip [2 is fastened, as by welding, to the current inlead wire I! andthe strip [3 i similarly mounted on the current inlead wire :8. Saidwires 11 and I8 pass through and make a hermetic joint with the stem IS.The contacts l4 and l 5 are separated a distance of approximately 5 mm.when the strips 12 and 13 are cold and engage each other when the stripsare heated by a glow discharge therebetween. When the contacts I4 and I5are thus engaged, the discharge in th switch is extinguished and after alapse or a short interval of time, depending on the thermal capacity ofthe bimetallic strips 12 and t3, the contacts are moved away from eachother by the flexing of the strips due to the cooling thereof.

When switches having this structure contain no radioactive material, thestarting potential thereof in the light is approximately 1-60 volts. Thestarting potential rises considerably in the dark and is frequentlyhigher than the voltage of th current source which is usuallyapproximately 220 volts. When a small quantity of an alpha-ray emittingmaterial, such as approxi mately 20 milligrams of uranium oxide, isdistributed on the interior surface of the envelope, the startingpotential is about 160 volts in the dark even after the switch has beenin complete darkness for aweek. The starting potential of l the switchin the light is the same whether it contains radioactive material ornot. The switch is thus highly useful for starting discharge lamps ofthe type having electrodes which require preheating.

I prefer to apply the uranium oxide to the inner surface of the envelope"directly above the contacts M and I5 as indicatedxat I9 in the drawing.Known methods of applying materials to glass surfaces are used. Asatisfactory method comprises the steps of mixing fine particles ofuranium oxide with a solution of nitrocellulose in amyl-acetate andapplying the suspension thus produced to the desired areaoftheenvelope). The nitrocellulose and amyl-acetate arethen removed fromthe envelope l0 in the usual 'manner during the manufacture of the.

switch'to' leave the'uranium oxide on the surface of the glass above thecontacts lland i5. While I have shown and described'my invention inconnection with one speciesof a glowtype thermal switch, I contemplatethe use thereof in switches of different structure. For

example, radioactive material may be used-for the purposes of theinvention in switches of the type disclosed in the copending applicationof Wayne C; Smitley, Serial No; 289,397, filed August 12, 1939,comprising astationary electrode and a bimetallic electrode constructed.and arranged to extinguish the discharge therebetween when saidelectrodes are heated by said discharge. The above applicationsare-assigned to the assignee of the present application. I contemplate,further, that the invention is useful in conjunction with switcheshaving other ionizable gases therein, such as neon, argon, xenon, orkrypton, or mixtures of such'gases, at suitable pressures; and inconjunction with switches having electrodes comprising .a metalsupportand material. of lowerwork function, such as magnesium, zinc, oralkaline earth compounds,

on said support to obtain a desired starting potential.

"In accordance with my copending application Serial No. 385,003, filedMarch 24, 1941, the envelope] IU- may consist'of uranium glass or theenvelope is made of any of the uranium-free glasses used commerciallyfor discharge devices, when desired, and the uranium glass is mounted insaid envelope It. In such a structure the uranium glass is preferably inthe form of a sleeve mounted about the electrodes thereof.'

Letters Patent of the United- States is:

A glow-type thermalswitoh comprising 'a sealed envelope containingheliummixed with 0.15 per cent argon at 'a pressure of about manln, electricalcontacts and radioactive mate'- ri a 1 k I BASIL NOEL CLACK.

stein 1-6 of uranium I

